1. Field of the Invention
The invention relates to an exhaust gas purification device of an engine.
2. Description of the Related Art
An exhaust gas purification device is known, which comprises a catalyst for purifying exhaust gas at the predetermined temperature range of the catalyst. For example, Japanese Unexamined Patent Publication No. 63-283727 discloses an exhaust gas purification device, which comprises a catalyst for purifying NO.sub.x in exhaust gas discharged from a diesel engine. The NO.sub.x catalyst reacts NO.sub.x in exhaust gas with HC to produce N.sub.2, CO.sub.2 and H.sub.2 O in order to purify NO.sub.x.
The NO.sub.x catalyst may purify exhaust gas only within a predetermined temperature range thereof. FIG. 30 shows a relation between the temperature of the catalyst and the purification ratio for NO.sub.x. As shown in FIG. 30, the NO.sub.x catalyst starts to purify exhaust gas at about 120.degree. C., and from about 120.degree. C., the purification ratio for NO.sub.x is increased as the temperature of the catalyst is increased until at about 230.degree. C. The purification ratio for NO.sub.x is decreased as the temperature of the catalyst is decreased from about 230.degree. C., and finally the catalyst does not purify exhaust gas at about 350.degree. C.
The reason why the purification ratio of the catalyst is changed as above described is that an amount of HC to be reacted with NO.sub.x is decreased since the reaction between HC and O.sub.2 becomes greater than the reaction between NO.sub.x and HC above a certain high temperature of the catalyst.
Therefore, to obtain a high purification ratio for NO.sub.x in the prior art, the temperature of the catalyst should be controlled to maintain the temperature of the catalyst within the temperature range at which the catalyst purifies exhaust gas, in particular, within the optimum temperature range at which the purification ratio for NO.sub.x is highest.
The temperature of the catalyst is subject to heat produced by the reaction between NO.sub.x and HC, and between HC and O.sub.2, and derived from exhaust gas. Therefore, in order to maintain the temperature of the catalyst within the temperature range at which the catalyst purifies exhaust gas, in particular, within the optimum temperature range at which the purification ratio for NO.sub.x is highest, it is necessary to control the temperature of the catalyst, for example, in such a manner that heating and cooling devices is arranged at the upstream area of the catalyst, and the heating device is operated when the temperature of the catalyst is lower than the above optimum temperature range while the cooling device is operated when the temperature of the catalyst is higher than the above optimum temperature range. However, the reaction in the catalyst and the temperature of exhaust gas are changed depending on the condition of the engine. Further, the reaction occurs locally in the catalyst. Therefore, it is complicated and difficult to control the temperature of the catalyst to maintain the temperature of the catalyst within the small optimum temperature range.